Control Apparatus For Vehicle

ABSTRACT

In a control apparatus for a vehicle having an electric motor for driving the wheels of the vehicle, an electric power source for energizing the electric motor, a motor torque target value calculation unit for controlling the electric power source, and a field current target value calculation unit, the motor field current is momentarily decreased when the difference between the actual motor armature current and the motor armature current target value exceeds a predetermined value or when the wheels are deemed to slip, and the motor field current is increased as the actual armature current of the motor substantially follows the motor armature current target value.

BACKGROUND OF THE INVENTION

This invention relates to a control apparatus for a vehicle having anelectric motor for driving the wheels of the vehicle and an electricpower supply for energizing the electric motor, and more particularly tothe control of the motor field current of the electric motor.

Japanese patent documents such as JP-A-2005-186756, Japanese Patent No.3610972, and Japanese Patent No. 3594024, disclose conventionalprocedures for controlling the motor field current in an electric motor,the procedure being used for a control apparatus comprising an electricmotor for driving the wheels of a vehicle, a motor field current controlunit for controlling the motor field current in the motor, an electricpower supply for energizing the electric motor, and a power control unitfor controlling the power supplied to the electric motor.

The document JP-A-2005-186756 discloses a procedure wherein the backelectromotive force induced in the electric motor is suppressed to yielda sufficient torque at high revolution speed operations through the“weak field control” that the field current of the motor is graduallydecreased as the revolution speed of the motor increases exceeding apredetermined threshold value.

The Japanese Patent No. 3610972 teaches a procedure wherein the motorfield current in the motor is decreasingly corrected so as to decreasethe back electromotive force generated in the motor when the generatoris deemed to generate insufficient power due to the change intransmission gear ratio, so that power generation in the generator isrecovered and the required torque can be generated.

According to the document Japanese Patent No. 3594024, the motor fieldcurrent in the motor is decreased in accordance with the decrease in themotor torque; the rate of decreasing the motor field current isincreased for smaller limit of power generation; and therefore the backelectromotive force induced in the motor is suppressed so that aneffective voltage can be obtained to produce current for obtaining atarget motor torque.

SUMMARY OF THE INVENTION

However, the document JP-A-2005-186756 only considers the increase inthe back electromotive force due to the increase in the revolution speedof the motor, and ignores the insufficiency of power generation. Aproblem, therefore, arises that a desired motor torque cannot beobtained due to the insufficiency of power generation.

On the other hand, according to the Japanese Patent No. 3610972 andJapanese Patent No. 3594024, the back electromotive force of the motoris lowered depending only on the insufficiency of power generation, butthe heat generation in the motor is not considered. Consequently, theback electromagnetic force of the motor remains low even when the powergeneration is recovered. As a result, large armature current continuesto flow for a certain period of time so that the motor is heated. Andthe worst result would be that the motor is broken.

Moreover, none of the documents JP-A-2005-186756, Japanese Patent No.3610972, and Japanese Patent No. 3594024 mention the delay of change inpower generation. Without a countermeasure for coping with the delay ofchange in power generation, a problem would arise that when the wheelsdriven by the motor slip, the moment to start decreasing the motortorque is delayed so that the slip cannot be quickly suppressed.Further, there would be a problem that the motor torque falls lowmomentarily when the power generation becomes insufficient momentarilydue to the depression in the revolution speed of the generator at thetime of speed change.

The object of this invention is to provide a control apparatus which canquickly generate a desired torque while suppressing the heat generationin the motor to the minimum.

According to this invention, which has been made to solve the abovementioned problems, there is provided as a first embodiment a controlapparatus for a vehicle, comprising an electric motor for driving thewheels of the vehicle, an electric power supply for energizing theelectric motor, and a control unit consisting mainly of a motor torquetarget value calculation unit for controlling the electric power supplythat energizes the electric motor and a motor field current target valuecalculation unit for calculating a target value for the motor fieldcurrent of the electric motor, wherein the control unit momentarilydecreases the field current of the electric motor when the differencebetween the actual armature current of the electric motor and the targetvalue for the motor armature current exceeds a predetermined value,while when the actual armature current roughly follows its target value,the motor field current is increased.

According to this invention, there is provided as a second embodiment, acontrol apparatus for a vehicle, comprising an electric motor fordriving the wheels of the vehicle, an electric power supply forenergizing the electric motor and a control unit consisting mainly of amotor torque target value calculation unit for controlling an electricpower supply that energizes the electric motor and a motor field currenttarget value calculation unit for calculating a target value for themotor field current of the electric motor, wherein the control unitmomentarily decreases the motor field current of the electric motor whenthe wheels of the vehicle are found to slip, while when the actualarmature current roughly follows its target value, the motor fieldcurrent is increased.

According to this invention, there is provided as a third embodiment acontrol apparatus for a vehicle as mentioned above as the firstembodiment of this invention, wherein the motor field current targetvalue calculation unit incorporates therein a motor current target valuecalculation unit; and the motor current target value calculation unitcomprises plural units for outputting different target values for themotor field current, a motor field current control mode judgment unitand a motor field current target value switching unit; and wherein themotor field current target value switching unit selects one of thedifferent motor field current target values in response to the output ofthe motor field current control mode judgment unit.

According to this invention, there is provided as a fourth embodiment acontrol apparatus for a vehicle as mentioned above as the thirdembodiment of this invention, wherein the plural units output pluraltarget values for the motor field current of the motor on the basis ofthe inputs selected from among the revolution speed of the motor, themotor armature current, the motor torque target value, the suppliedvoltage and the previous target value for the motor field current.

According to this invention, there is provided as a fifth embodiment acontrol apparatus for a vehicle as mentioned above as the thirdembodiment of this invention, wherein the plural units comprises a firstoutput unit for outputting a first motor field current target valuedepending on the revolution speed of the motor; a second output unit foroutputting a second motor field current target value depending on themotor torque target value and the motor armature current; a third outputunit for outputting a third motor field current target value dependingon the motor armature current, the revolution speed of the motor and thesupplied voltage; and a fourth output unit for outputting a fourth motorfield current target value depending on the previous target value forthe motor field current.

Further, according to this invention, there is provided a controlapparatus for a vehicle wherein the rate of increase in the motor fieldcurrent is kept equal to or lower than the rate of change in thesupplied power.

Furthermore, according to this invention, there is provided a controlapparatus for a vehicle wherein the electric power supply is a generatorand the supplied voltage is the voltage generated by the generator.

A control apparatus for a vehicle according to this invention canquickly generate a required torque while suppressing the heat generationin the motor to the minimum, by controlling the motor field current inaccordance with the supplied power and making the motor field current aslarge as possible within a range for which the required torque can beobtained, as a delay is caused in the change in the supplied power whenthe motor torque is changed in response to the insufficiency of powersupplied from such an electric power supply as a generator or to theslip of the wheels driven by the electric motor. Thus, the controlapparatus of this invention can keep the slip of the wheels driven bythe electric motor within a small extent and therefore hill climbingperformance on a slippery road can be improved. Also, as the maximumtorque can be generated even when supplied power is insufficient,accelerating performance can be improved.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified structure of a vehicle to which a controlapparatus as the embodiment of this invention is applied;

FIG. 2 shows in block diagram a 4WD controller as an embodiment of thisinvention;

FIG. 3 shows in block diagram a field current target value calculationunit in the 4WD controller as shown in FIG. 2;

FIG. 4 shows in block diagram a motor current target value calculationunit in the 4WD controller as shown in FIG. 2;

FIG. 5 is a mode transition diagram for a motor field current controlmode judgment unit for use in the 4WD controller as shown in FIG. 2;

FIG. 6 is a table listing the conditions for judging the transition ofcontrol modes, for use with the 4WD controller as shown in FIG. 2 a;

FIG. 7 graphically shows the results of test run along a slipperyascending slope of a vehicle with a conventional control apparatusmounted thereon;

FIG. 8 graphically shows the results of test run along a slipperyascending slope of a vehicle with a control apparatus as shown in FIG. 2mounted thereon;

FIG. 9 graphically shows the results of test run along a frictional flatroad of a vehicle with a conventional control apparatus mounted thereon;and

FIG. 10 graphically shows the results of test run along a frictionalflat road of a vehicle with a control apparatus as shown in FIG. 2mounted thereon.

DESCRIPTION OF THE EMBODIMENTS

A control apparatus for a vehicle as a first embodiment of thisinvention will now be described with reference to the attached drawings.

FIG. 1 shows a simplified structure of a vehicle (electric 4-wheel-drivevehicle) to which this invention is applied. The vehicle to which thisinvention has been applied has an engine 1, a transmission 2 and agenerator 3, mounted thereon, and the mechanical power generated by theengine 1 is transmitted to the transmission 2 and the generator 3. Thetransmission 2, after being adjusted to a desired speed setting,transmits the mechanical power to the front axles 4 a and 4 b to drivethe associated front wheels 5 a and 5 b, respectively. The mechanicalpower transmitted to the generator 3 is converted to electric power,which is supplied via a power line 6 to an electric motor 7. Theelectric motor 7 is a field-control DC motor which converts the suppliedelectric power into mechanical torque. The mechanical torque is thendistributed, by means of a rear differential gear unit 8, to the rearaxles 4 c and 4 d to drive the rear wheels 5 c and 5 d, respectively.Wheel speed sensors 9 a, 9 b, 9 c and 9 d are mounted near the front andrear wheels 5 a, 5 b, 5 c and 5 d, respectively. The wheel speed sensorsdetect the wheel speeds of the respective wheels. The vehicle to whichthis invention has been applied has a 4WD controller 100 mountedthereon. The 4WD controller 100 receives signals representing thecorresponding wheel speeds from the wheel speed sensors 9 a, 9 b, 9 cand 9 d. These wheel speed signals are used to control the generator 3and the electric motor 7.

FIG. 2 systematically shows in block diagram a 4WD controller as a firstembodiment of this invention. The 4WD controller 100, as shown alreadyin FIG. 1, comprises a motor torque target value calculation unit 110and a field current target value calculation unit 120. The 4WDcontroller 100 receives as its inputs the wheel speed signals from thewheel speed sensors 9 a, 9 b, 9 c and 9 d, the generator revolutionspeed signal and the generator voltage signal from the generator 3, andthe motor revolution speed signal and the motor voltage signal from theelectric motor 7. The generator voltage signal and the motor voltagesignal are the signals representing the armature voltages of thegenerator 3 and the motor 7, respectively. Further, the 4WD controller100 delivers the generator field current control signal to the generator3 and the motor field current control signal to the motor 7. Generally,the motor torque can be calculated as the product of the motor torquecoefficient determined by the motor field current and the currentflowing through the armature winding of the motor. Hence, thefield-control DC motor can control the torque coefficient by controllingthe field current of the motor. Accordingly, the 4WD controller 100 onone hand delivers the generator field current control signal, whichcontrols the generated output of the generator 3, and on the other handdelivers the motor field current control signal, which controls themotor field current for controlling the torque of the motor 7.

The motor torque target value calculation unit 110 determines the motortorque target value MTt on the basis of the difference between theaverage VWF of the wheel speeds of the right and left front wheels andthe average VWR of the wheel speeds of the right and left rear wheels.As the value obtained by subtracting the rear wheel speed average VWRfrom the front wheel speed average VWF becomes larger, the motor torquetarget value MTt is made larger accordingly. The motor torque targetvalue calculation unit 110 incorporates therein a rear wheel slipdetector, which sets a rear wheel slip flag R_SLIP when the rear wheelsslip and unsets the rear wheel slip flag R_SLIP when the rear wheelsstop slipping.

FIG. 3 shows in block diagram a field current target value calculationunit 120 in the 4WD controller as shown in FIG. 2.

The field current target value calculation unit 120 detects the delay ofchange in or the insufficiency of, the generated power depending on thegenerated voltage GHV. This unit 120 further controls the generatorfield current Ig and the motor field current If so that the motor 7 canproduce the motor torque Tm adapted to the motor torque target value MTtby controlling the motor torque coefficient and the back electromotiveforce Em of the motor 7 through the control of the field current of thehighly responsive motor 7 even when there is the delay of change in orthe insufficiency of, the generated power.

As shown in FIG. 3, the field current target value calculation unit 120comprises a motor current target value calculation unit 130, a generatorfield current target value calculation unit 140 and a sampling ratedelay circuit B1. The motor current target value calculation unit 130calculates the motor armature current target value Iat and the motorfield current target value Ift from the motor torque target value MTt,the motor revolution speed Nm, the voltage GHV generated by thegenerator 3 (also referred to as generator voltage GHV), the motorvoltage MHV, the rear wheel slip flag R_SLIP and the previous generatorfield current target value Igt_z. The structure of the motor currenttarget value calculation unit 130 will be described later. The generatorfield current target value calculation unit 140 calculates the generatorfield current target value Igt from the generator revolution speed Ngand the motor armature current target value Iat by using a map. Thesampling rate delay circuit B1 receives as its input the generator fieldcurrent target value Igt and delivers as its output the previousgenerator field current target value Igt_z after a sampling period.

FIG. 4 shows in block diagram the motor current target value calculationunit 130 shown in FIG. 3.

As shown in FIG. 4, the motor current target value calculation unit 130incorporates therein an armature current target value calculation unit131, an armature current calculation unit 132, a differentiator 133, afield current maximizing controller 134, a torque optimizing controller135, a torque maximizing controller 136, a motor field current increasecontroller 137, a motor field current control mode judgment unit 150, amotor field current target value switching unit 138 and a sampling ratedelay circuit B2.

The armature current target value calculation unit 131 calculates themotor armature current target value Iat from the previous motor fieldcurrent target value Ift_z and the motor torque target value MTt byusing a map. The armature current calculation unit 132 calculates themotor armature current Ia from the motor voltage MHV and the generatorvoltage GHV by using the following expression (1).

$\begin{matrix}{{I\; a} = \frac{{G\; H\; V} - {M\; H\; V}}{R\; 1}} & (1)\end{matrix}$

, where R1 indicates the resistance value of the conductor (i.e. powerline 6) connecting the generator 3 and the motor 7.

The differentiator 133 calculates the differential value GHV_d of thegenerator voltage GHV. The field current maximizing controller 134calculates the first motor field current target value Ift1 from themotor revolution speed Nm by using a map. This map used by the fieldcurrent maximizing controller 134 is that which is used to perform suchweak field control as disclosed in the afore mentioned documentJP-A-2005-186756.

The torque optimizing controller 135 calculates the motor torquecoefficient target value KIf2 from the motor armature current Ia and themotor torque target value MTt by using the following expression (2).

$\begin{matrix}{{K\; I\; f\; 2} = \frac{\; {M\; T\; t}}{I\; a}} & (2)\end{matrix}$

The torque optimizing controller 135 also calculates the second motorfield current target value Ift2 p (not shown) before correction from themotor armature current Ia and the motor torque coefficient target valueKIf2 by using a map. The torque optimizing controller 135 finallyoutputs as the second motor field current target value Ift2 the valueobtained by adding a predetermined value c_Ift2 (not shown) to thesecond motor field current target value Ift2 p (not shown) beforecorrection. The predetermined value c_Ift2 is pre-adjusted in such amanner that the change in the supplied electric power can smoothlyfollow the increase in the field current of the motor 7.

The torque maximizing controller 136 calculates the motor torquecoefficient target value KIf3 from the motor revolution speed Nm and thegenerator voltage GHV by using the following expression (3). In theexpression (3), the coefficient C is the value obtained by dividing theback electromotive force Em of the motor by the motor torque coefficientdetermined on the basis of the motor revolution speed Nm and the motorfield current. The coefficient C is a constant that should be previouslyobtained through calculation.

$\begin{matrix}{{K\; I\; f\; 3} = \frac{G\; H\; V}{2 \times C \times {Nm}}} & (3)\end{matrix}$

The torque maximizing controller 136 calculates the third motor fieldcurrent target value Ift3 from the motor armature current Ia and themotor torque coefficient target value KIf3 by using a map.

The motor field current increase controller 137 outputs as the fourthmotor field current target value Ift4 the value obtained by adding thepredetermined value c_Ift4 to the previous motor field current targetvalue Ift_z. The predetermined value c_Ift2 is pre-adjusted in such amanner that the change in the supplied electric power can smoothlyfollow the increase in the field current of the motor 7.

The motor field current control mode judgment unit 150 decides on thecontrol mode on the basis of the rear wheel slip flag R_SLIP, theprevious generator field current target value Igt_z, the previous motorfield current target value Ift_z, the motor armature current targetvalue Iat, the motor armature current Ia, the generator voltagedifferential value GHV_d, and the first motor field current target valueIft1. The mode switching condition for the motor field current controlmode judgment unit 150 will be described later.

The motor field current target value switching unit 138 outputs as themotor field current target value Ift one of the first motor fieldcurrent target value Ift1, the second motor field current target valueIft2, the third motor field current target value Ift3 and the fourthmotor field current target value Ift4 in accordance with the controlmode determined by the motor field current control mode judgment unit150. In order for the motor field current target value to be preventedfrom changing rapidly and largely, for example, when the third motorfield current target value Ift3 is switched to the second motor fieldcurrent target value Ift2, the change in the motor field current targetvalue Ift may be smoothed by means of a low-pass filter or its rate maybe so limited as to become less than a predetermined value by means of arate limiter.

The sampling rate delay circuit B2 receives the motor field currenttarget value Ift and delivers it as the previous motor field currenttarget value Ift_z after a sampling period.

According to the above described procedure, the motor current targetvalue calculation unit 130 calculates the motor armature current targetvalue Iat and the motor field current target value Ift.

FIG. 5 is a mode transition diagram for the motor field current controlmode judgment unit 150 for use in the 4WD controller as the firstembodiment of this invention. The control mode is divided into the rearwheel grip mode 151 and the rear wheel slip mode 152. The rear wheelgrip mode 151 consists of the usual (or normal) mode 153 and thegeneration insufficiency mode 154. The rear wheel slip mode 152 consistsof the field current increasing mode 157 and the field currentdecreasing mode 158. The generation insufficiency mode 154 furtherconsists of the generation delay mode 155 and the generation limitingmode 156.

The motor field current target value Ift as the output of the motorfield current target value switching unit 138 assumes the first motorfield current target value Ift1 in the usual (or normal) mode 153, thesecond motor field current target value Ift2 in the generation delaymode 155 and the field current decrease mode 158, the third motor fieldcurrent target value Ift3 in the generation limiting mode 156, and thefourth motor field current target value Ift4 in the field currentincreasing mode 157.

FIG. 6 is a table listing the conditions for judging the transition ofcontrol modes, for use with the 4WD controller as the first embodimentof this invention. Initially, the usual (or normal) mode 153 is started.In the usual (or normal) mode 153, if the difference between the motorarmature current target value Iat and the motor armature current Ia isnot smaller than a predetermined threshold t_Iat1 and if the generatorfield current target value Igt is not greater than a predeterminedthreshold t_Igt1, the judgment is of generation delay so that thetransition from the usual (or normal) mode 153 to the generation delaymode 155 takes place. Also in the usual (or normal) mode 153, if thegenerator voltage differential value GHV_d is not greater than apredetermined threshold t_GHV_d, or if the difference between the motorarmature current target value Iat and the motor armature current Ia isnot smaller than a predetermined threshold t_Iat2 and if the generatorfield current target value Igt is not smaller than a predeterminedthreshold t_Igt2, then the judgment is of generation limit so that thetransition from the usual (or normal) mode 153 to the generation limitmode 156 takes place. It is assumed here that the threshold t_Iat2 isgreater than the threshold t_Iat1 and that the threshold t_Igt2 isgreater than the threshold t_Igt1.

In the generation delay mode 155, if the judgment is of generationlimit, the transition from the generation delay mode 155 to thegeneration limit mode 156 takes place. In the generation limit mode 156,if the difference between the motor armature current target value Iatand the motor armature current Ia is smaller than a predeterminedthreshold t_Iat3, the judgment is of generation recovery so that thetransition from the generation limit mode 156 to the generation delaymode 155 takes place. In the generation insufficiency mode 154, if themotor armature current Ia is not smaller than the motor armature currenttarget value Iat and if the motor field current target value Ift is notsmaller than the first motor field current target value Ift1, then thejudgment is of generation properness so that the transition from thegeneration insufficiency mode 154 to the usual (or normal) mode 153takes place.

If the rear wheel slip flag R_SLIP is set, the transition from the rearwheel grip mode 151 to the field current decrease mode 158 in the rearwheel slip mode 152 takes place. In the field current decrease mode 158,if the difference between the motor armature current Ia and the motorarmature current target value Iat is not greater than a predeterminedthreshold t_Iat4, the judgment is of generation insufficiency so thatthe transition from the field current decrease mode 158 to the fieldcurrent increase mode 157 takes place. In the field current increasemode 157, if the difference between the motor armature current Ia andthe motor armature current target value Iat is not smaller than apredetermined threshold t_Iat5, the judgment is of generation excess sothat the transition from the field current increase mode 157 to thefield current decrease mode 158 takes place. Also in the field currentincrease mode 157, if the motor field current target value Ift is notsmaller than the first motor field current target value Ift1 and if therear wheel slip flag R_SLIP has been unset (i.e. equal to zero), thenthe judgment is of field current recovery so that the transition fromthe field current increase mode 157 to the usual (or normal) mode 153takes place.

As described above, the motor field current target value Ift can bedetermined by switching among the control modes.

The technical advantage of this invention will now be described withreference to FIGS. 7 through 10.

FIGS. 7 and 8 graphically show the results of test runs along a slipperyascending slope of vehicles with a conventional control apparatus and acontrol apparatus as shown in FIG. 2 mounted thereon, respectively. FIG.7 shows the results of test run of a vehicle with a control apparatusdisclosed in JP-A-2005-186756 mounted thereon, and FIG. 8 shows theresults of test run of a vehicle with a control apparatus according tothis invention mounted thereon. Reference should be made to the graphrepresenting the rear wheel speed VWR, which indicates that the rearwheels driven by the motor slip due to the low friction coefficient ofthe road surface. When the rear wheels slip, the motor torque targetvalue MTt decreases to suppress the slip of the rear wheels.Accordingly, the power generated by the generator decreases to lower thegenerator voltage GHV. The depression of the generator voltage GHVcauses the decrease in the motor armature current Ia so that the motortorque Tm decreases.

In FIG. 7, it is seen that as the decrease in the generator voltage isdelayed, the decrease in the motor torque Tm is also delayed. In FIG. 8,on the other hand, the motor torque Tm is decreased without delay withrespect to the motor torque target value MTt by decreasing the motorfield current If to a great extent. As a result, the slip can be limitedto a small extent. Also, when the generator voltage is loweredsufficiently, the motor field current If is increased accordingly sothat excess current can be prevented from flowing through the motorarmature, preventing the excess heating of the motor. As shown in FIG.7, too, the motor field current If decreases to a small extent duringthe slip. This decrease, however, is simply caused by the decrease inthe motor field current If as a result of the weak field control asdisclosed in JP-A-2005-186756 due to the increase in the motorrevolution speed Nm during the slip. The decrease does not make themotor torque Tm follow the motor torque target value MTt.

FIGS. 9 and 10 graphically show the results of simulation test runs of avehicle along a highly frictional, flat road. FIG. 9 shows the resultsof simulation test run along a frictional flat road of a vehicle with aconventional control apparatus as disclosed in JP-A-2005-186756 mountedthereon and FIG. 10 shows the results of simulation test run along africtional flat road of a vehicle with a control apparatus according tothis invention mounted thereon. The graphs representing the gearpositions GP in both FIGS. 9 and 10 indicate that speed change takesplace in a little more than 5 seconds. Accordingly, the generatorrevolution speed Ng decreases, resulting in the decrease in the powergeneration leading to the decrease in the generator voltage GHV. FIG. 9shows the situation where the motor torque Tm cannot be generated sincethe armature current Ia is interrupted according as the differencebetween the generator voltage GHV and the back electromotive force Em ofthe motor vanishes after the speed change. As shown in FIG. 10, on theother hand, the motor field current If is decreased in response to thedepression of the generator voltage GHV, the back electromotive force Emof the motor is accordingly decreased, and consequently the smooth motortorque Tm following the motor torque target value MTt can be generated.After the lapse of 10 seconds, the motor field current If is decreasedin response to the increase in the generator voltage GHV. Accordingly,the armature current Ia is prevented from flowing excessively and theoverheating of the motor is prevented.

From the result of the simulation described above, it is known that thecontrol apparatus according to this invention can quickly develop adesired motor torque while preventing the overheating of the motor dueto excess current flowing through the armature winding.

In the above described embodiment of this invention, description is madeof a case where the front wheels 5 a and 5 b are driven by the engine 1while the rear wheels 5 c and 5 d are driven by the electric motor 7.However, a variation may be possible wherein front wheels are driven byan electric motor and rear wheels are driven by an engine.

In the above described embodiment of this invention, the 4WD controller100 serves as both the motor field current control unit and the electricpower supply control unit and the generator 3 is used as the electricpower supply. A variation may also be realized wherein a battery, acapacitor or a fuel cell is used as an electric power supply.

In the above described embodiment of this invention, the field-controlDC motor is used as the motor 7 and the power generated by the generatoris fed directly to the motor 7 via the power line 6. In a variation ofthis embodiment, a field-control AC motor may be used and this AC motorcan be driven by means of an inverter connected between the power lineand the AC motor.

In the above described embodiment of this invention, the motor torquetarget value calculation unit 110 is used which determines the motortorque in response to the difference between the front wheel speed andthe rear wheel speed. In a modified embodiment, a motor torque targetvalue setting unit may be used which receives an accelerator actuationsignal to determine the torque of the motor 7 in accordance with thedegree of the accelerator actuation.

In the above described embodiment of this invention, the motor fieldcurrent control mode judgment unit 150 is used which detects theinsufficiency of power generation on the basis of the generator voltagedifferential value GHV_d, the motor armature current target value Iat,and the motor armature current Ia. However, the insufficiency of powergeneration may be detected by judging from the generator revolutionspeed. Also, the insufficiency of power generation may be detected inview of the engine revolution speed, the wheel speed, the gear ratio ofthe transmission, or the front differential gear ratio.

In the above described embodiment of this invention, the motor torquecoefficient target value is calculated from the generator voltage, themotor armature current and the motor revolution speed and then the motorfield current is controlled through the calculation of the motor fieldcurrent target value by using a map. Instead, a motor field currentcontrol unit may be used which gradually decreases the motor fieldcurrent when the judgment of power insufficiency is made. Or a motorfield current control unit may be used which controls the motor fieldcurrent through the calculation of the motor field current target valueby using a map on the basis of generator voltage, motor armaturecurrent, motor revolution speed and generator revolution speed.

In the above described embodiment of this invention, the predeterminedthresholds c_Ift2 and c_Ift4 used by the motor field current increasecontroller 137 are so preset that the change in the supplied electricpower can smoothly follow the increase in the field current of the motor7. The thresholds c_Ift2 and c_Ift4, however, may be varied in responseto the absolute value of the speed of the change in the suppliedelectric power. Or the thresholds c_Ift2 and c_Ift4 may be made greateraccording as the field current of the generator 3 becomes smaller.

In the above described embodiment of this invention, the motor fieldcurrent control unit is used wherein the delay of the change in thegenerator voltage or the generation insufficiency is detected on thebasis of the generator voltage, the motor armature current and the motorarmature current target value and the overheating of the motor isprevented by increasing the motor field current according as the delayor the insufficiency is being recovered. However, a motor field currentcontrol unit may be used wherein the temperature of the motor isdetected by a sensor and the overheating of the motor is prevented byincreasing the motor field current when the motor temperature reaches apreset value. Alternatively, a motor field current control unit may beused wherein the motor field current is increased on the basis of thejudgment that the electric resistance of the motor armature winding hasincreased due to the heat generation in the motor when the differencebetween the back electromotive force of the motor calculated from themotor voltage and the motor armature current and the back electromotiveforce of the motor calculated from the motor revolution speed and themotor field current, exceeds a preset threshold. Moreover, a motor fieldcurrent control unit may be used wherein the motor field current is notdecreased even when the motor temperature exceeds a preset threshold.

As described in relation to FIGS. 9 and 10, the embodiment of thisinvention can be advantageously used in the case where power generationbecomes insufficient due to speed change. The control apparatusaccording to this invention can also be used in the case where anelectric power supply also energizes another auxiliary device and thepower supply becomes insufficient when power consumed by the auxiliarydevice increases. In such a case, a power distribution control unit maybe used which restricts the power supply to the auxiliary device orpreferentially supplies power to the motor.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A control apparatus for a vehicle having an electric motor to drivethe wheels of the vehicle and an electric power supply to energize theelectric motor, comprising a control means which temporarily decreasesthe field current of the motor when the difference between the actualmotor armature current and the motor armature current target valueexceed a predetermined value, and which increases the field current ofthe motor according as the actual armature current of the motorsubstantially follows the motor armature current target value.
 2. Acontrol apparatus for a vehicle having an electric motor to drive thewheels of the vehicle and an electric power supply to energize theelectric motor, comprising a control means which temporarily decreasesthe field current of the motor when the front wheels slip, and whichincreases the field current of the motor according as the actualarmature current of the motor substantially follows the motor armaturecurrent target value.
 3. A control apparatus as claimed in claim 1,wherein the control means further comprising: a field current targetvalue calculation means, and wherein the field current target valuecalculation means comprises a motor current target value calculationmeans that comprises plural means for outputting different target valuesfor the motor field current, a motor field current control mode judgmentmeans, and a motor field current target value switching means, andwherein the motor field current target value switching means selectivelyoutputs one of the different target values in accordance with thejudgment result obtained by the motor field current control modejudgment means.
 4. A control apparatus as claimed in claim 2, whereinthe control means further comprising: a field current target valuecalculation means, and wherein the field current target valuecalculation means comprises a motor current target value calculationmeans that comprises plural means for outputting different target valuesfor the motor field current, a motor field current control mode judgmentmeans, and a motor field current target value switching means, andwherein the motor field current target value switching means selectivelyoutputs one of the different target values in accordance with thejudgment result obtained by the motor field current control modejudgment means.
 5. A control apparatus as claimed in claim 3, whereinsaid plural means for outputting different target values produce andoutput a plurality of target values for the motor field current based onan input which is selected among a revolution speed of the motor, anarmature current of the motor, a motor torque target value, a supplyvoltage and a previous motor field current target value.
 6. A controlapparatus as claimed in claim 4, wherein said plural means foroutputting different target values produce and output a plurality oftarget values for the motor field current based on an input which isselected among a revolution speed of the motor, an armature current ofthe motor, a motor torque target value, a supply voltage and a previousmotor field current target value.
 7. A control apparatus as claimed inclaim 3, wherein said plural means for outputting different targetvalues further comprises: means for outputting a first target value forthe motor field current based on the revolution speed of the motor;means for outputting a second target value for the motor field currentbased on the motor torque target value and the armature current of themotor; means for outputting a third target value for the motor fieldcurrent based on the armature current of the motor, the revolution speedof the motor and the supply voltage; and means for outputting a fourthtarget value for the motor field current based on the previous motorfield current target value.
 8. A control apparatus as claimed in claim4, wherein said plural means for outputting different target valuesfurther comprises: means for outputting a first target value for themotor field current based on the revolution speed of the motor; meansfor outputting a second target value for the motor field current basedon the motor torque target value and the armature current of the motor;means for outputting a third target value for the motor field currentbased on the armature current of the motor, the revolution speed of themotor and the supply voltage; and means for outputting a fourth targetvalue for the motor field current based on the previous motor fieldcurrent target value.
 9. A control apparatus as claimed in claim 1,wherein a rate of increase in the motor field current is kept equal toor lower than a rate of change in a supplied power.
 10. A controlapparatus as claimed in claim 2, wherein a rate of increase in the motorfield current is kept equal to or lower than a rate of change in asupplied power.
 11. A control apparatus as claimed in claim 1, whereinsaid electric power supply is a generator which produces a supplyvoltage supplied to the motor.
 12. A control apparatus as claimed inclaim 2, wherein said electric power supply is a generator whichproduces a supply voltage supplied to the motor.